(1) Field of the Invention
The present invention relates to an air-to-fuel ratio control system for an internal combustion engine, and, more particularly, to a system wherein the air-to-fuel ratio of the mixture is speedily returned to the predetermined ratio in response to an oxygen concentration in the exhaust gas.
(2) Description of the Prior Art
A closed-loop air-to-fuel ratio control system have been highly appreciated for purifying exhaust gases emitted from the internal combustion engine. Some of these techniques are disclosed in the patents U.S. Pat. No. 3,815,561 and U.S. Pat No. 3,874,171. In these techniques an oxygen responsive sensor disposed in an exhaust pipe and a feedback circuit connected thereto are employed to control either air amount or fuel amount of the mixture in response to the oxygen concentration in the exhaust gas. And it is a well-known art to integrate with the predetermined integration constant, a sensor output voltage which is shown in (A) of FIG. 1 and indicative of the oxygen existence (air-to-fuel ratio) and obtain an integrated voltage for controlling the air-to-fuel ratio in accordance therewith. Due to this control, the air-to-fuel ratio of the mixture to be supplied to the engine changes as shown in (B) of FIG. 1. This Figure depicts a limit cycle trajectory which results from the time delay during which the mixture is converted to the exhaust gas through combustion in the engine. This time delay results in the feedback delay of the system. The time delay is denoted in (B) of FIG. 1 as the time T.sub.1 and T.sub.2 in which the mixture is controlled to return to the stoichiometric ratio (.lambda.=1) from the least ratio (the richest ratio) and the greatest ratio (the leanest ratio), respectively. To precisely control the air-to-fuel ratio of the mixture to the stoichiometric ratio, the feedback delay is desired to be short.